Variation of grouped suspended sediment deposition and erosion in Shashi—Hankou reach after the construction of the Three Gorges Dam

The adjustment of scouring and silting downstream of reservoirs is a long-term complex process, which has a certain impact on flood protection, navigation and ecology. To explore the evolution trend of Shashi-Hankou reach in the middle reach of the Yangtze River and provide a reference for the prevention and control of adverse changes, the temporal and spatial changes of deposition and erosion of grouped suspended sediment after the construction of the Three Gorges Dam (TGD) are analyzed. After the construction of TGD, the sediments of each particle size group in Shashi-Jianli reach are eroded; and the 0.125-0.250 mm particle size group has the largest amount of sediment erosion when the discharge at Shashi station is 10 000-25 000 m³/s. Taking 0.125 mm as the boundary particle size of coarse and fine particles for the Jianli-Luoshan reach, the reach is characterized by "coarse deposition" in flood season and storage period, and "fine erosion" in low water season and pre-flood falling period. The sediments of each particle size group in the Luoshan-Hankou reach are eroded, and the larger the discharge, the greater the amount of erosion. On this basis, by establishing the relationship between discharge and sediment of different particle size groups and analyzing the sediment carrying capacity formula, it is found that after the construction TGD, the scouring and silting of the Shashi-Jianli reach and the Luoshan-Hankou reach are affected jointly by sediment and hydraulic factors, and the impact of sediment factors on the Shashi-Jianli reach is greater than that on the Luoshan-Hankou reach. At the same time, the "coarse deposition and fine erosion" in the Jianli-Luoshan reach is mainly affected by the recovery degree of sediment concentration, which is related to the scour replenishment of the upstream riverbed and the exchange of coarse and fine sediments in this reach, reflecting that the sediment carrying capacity of the river channel under scour condition can be increased by grading adjustment.